The present invention generally relates to an image forming apparatus having plural magnetic brush type developing devices arranged near the surface of a photosensitive member.
Conventionally, as one example of an image forming apparatuses as referred to above, there has been proposed an arrangement as described below. In the arrangement, plural magnetic brush type developing devices are detachably arranged at the side of a photosensitive member and a toner with different colors are accommodated in each of the devices, whereby the device with the toner in the color selected by an operator is selectively driven to form an image in black or the other color except for black.
In the apparatus, a developer including the toner and a carrier, which is held on the surface of a developing sleeve, contacts with the surface of the photosensitive member so as to develop an electrostatic latent image formed onto the surface of the member into a visible toner image by the magnetic brush type developing device. In the developing operation, since reproducibility RSI of density of a reproduced solid image conflicts with reproducibility RLI of density of a reproduced line image with each other, the higher reproducibility RSI of the density of the solid image results in the lower reproducibility RLI of the density of the line image, while the lower reproducibility RSI of the density of the solid image results in the higher reproducibility RLI of the density of the line image.
In a developing region where the photosensitive member confronts with the developing sleeve, if a large amount of the developer per hour contact the photosensitive member, the electrostatic latent image is developed to form a visual toner image with high density by supplying a sufficient amount of the toner thereto. However, since the toner which formed the toner image is simply and electrically put onto the surface of the photosensitive member, it is conceivable that the toner is scrapped off from the surface thereof by an external force such as a contact force of the magnetic brush. The toner put on an electrostatic latent image of a line image intends to easily scrapped off than that put on an electrostatic latent image of a solid toner image with some width. Therefore, it causes the following problem. That is, while the solid image and a normal letter image are reproduced with general faithful to the original imaged, the line image is reproduced with very lower density, some or all of the line image can not be reproduced.
On the other hand, in the developing region, if a small amount of the developer per hour contact the photosensitive member, a small amount of the toner is supplied to the photosensitive member and thus the toner is not sufficiently supplied to the electrostatic latent image. Then, the reproduced image is with lower density. However, it is hard to scrapped off the toner supplied to the electrostatic latent image therefrom by the magnetic brush. Even though the latent image to be developed includes a fine line image, the toner supplied to the latent image is transferred and fixed to a copy paper so that the original fine line image can be reproduced as a fine line image with lower density and with relatively faithful to the original image.
Then, the image forming apparatus employing the magnetic brush type developing devices has the problem that it is difficult to reproduce the line image as well as the solid image and the normal letter image while having sufficient density.
Moreover, generally, in image forming apparatuses having a magnetic brush type developing device, developer packaged density, toner content ratio, and toner charged electric potential at a portion confronting a photosensitive member with a developing sleeve cause the quality of a reproduced image to change. Here, the developer packaged density F at a developing region where a developing sleeve confronts with a photosensitive member is shown by the equation which is EQU F=Vd / Ds,
where Vd is an amount of a developer passed through the region per hour and Ds is a gap between the sleeve and the member. Then, for example, higher developer packaged density results in improvement of reproducibility RSI of density of a reproduced solid image. Conversely, lower developer packaged density results in improvement of reproducibility RLI of density of a reproduced line image. However, as described previously, the reproducibility RSI of the density of the solid image conflicts with the reproducibility RLI of the density of the line image, and then it was impossible for both of the reproducibility RSI and RLI of the density of the solid image and the line image to simultaneously improve by only one developing device.
In addition, the amount of toner consumption in a developing device for reproducibility RSI of the density of a reproduced solid image is much larger than that in a developing device for reproducibility RLI of the density of a reproduced line image. Then, there is the problem that it is unreasonable that the developer capacity in each device is the same in design but it is necessary for the device which consumes large amounts of the developer to accommodate such an amount of developer.
Furthermore, conventionally, there have been proposed image forming apparatuses for circularly transporting a two-component developer composed of a toner and a carrier. In these apparatuses, a developing sleeve is arranged inside a housing to confront with a photosensitive member, a partition is arranged in a back space of the sleeve by which plural transporting paths are approximately parallel with each other are formed, and connecting paths for connecting both the transporting paths with each other at ends of the housing are arranged at a position adjacent to the partition in the housing, thus the developer is circularly transported by a transporting means arranged in each transporting path to supply the developer to the surface of the sleeve.
However, according to these apparatuses, since the toner is graduately consumed while being transported from the upstream side in a transporting direction of the transporting path adjacent to the sleeve to the downstream side thereof, the toner density in the developer, that is, the toner mixing ratio to the carrier, becomes lower while the toner is transported toward the downstream therein. Then, when the toner is replenished in the transporting path, ununiformity of the toner density in consuming the toner overlaps ununiformity of the toner density in replenishing the toner to magnify the ununiformity thereof.
Therefore, the density of the developer at the upstream side of the transporting path adjacent to the sleeve becomes higher and conversely the density at the downstream side thereof becomes lower. Then, these states results in that an image at one side of an original document is reproduced with higher density than necessary while producing any fog, and conversely, an image at the other side thereof is reproduced with lower density.